Liquid cooled casting drums



Aug. 4, 1964 w. PABO LIQUID COOLED CASTING yDRUMS Filed Jan. 16. 1961 6 Fly'. 3 WALDEMAR PABO ATTORNEYS v www distorting the casting surface.

United States Patent O "ice 3,142,866 LIQUID CGOLED CASTING DRUMS Waidemar Pabo, Rochester, NX., assigner to Eastman Kodak Company, Rochester, NX., a corporation of New Jersey Filed lan. 16, 1961, Ser. No. 83,056 S Claims. (Cl. 18-15) The present invention relates to liquid cooled casting drums or chill rolls used in forming a continuous sheet or film by the melt casting process, in which a thermoplastic material in molten form is uniformly applied to the cylindrical surface of the casting drum and is cooled by the drum to form a continuous sheet or lm. More particularly, the invention relates to an arrangement for controlling the ow of heat within such a drum to maintain a highly uniform temperature throughout the drum under operating conditions.

Such casting drums are commonly made in the form of a closed cylinder and are cooled by completely filling the interior of the drum with a circulating liquid, usually water, the temperature of the casting surface of 'the drum being controlled by regulating the temperature and/or the rate of flow of the coolant. Since heat is constantly being applied to the cylindrical surface of such a drum by the molten material, the cylindrical casting surface has in the past been heated to a higher temperature than the end surfaces. This is particularly true in the case of large drums constructed of a material such as stainless steel which has a relatively low coefficient of thermal conductivity. Furthermore, a temperature gradient also occurs in such drums across the casting surface itself because the heat applied to the casting surface by the molten material along the edges of the strip or sheet will be dissipated to some extent by conduction to the ends of the drum, resulting in a lower casting surface temperature at the edges of the strip than at the center. These temperature variations throughout a substantially homogeneous drum induce undesirable stresses and distortions in the drum.

While these various distortional stresses induced in the drum by such temperature Variations are interrelated and not capable of simple resolution, the following exam- .ples will serve to illustrate their effect upon the casting surface. If the temperature of a casting drum comprising a tubular member joined at its ends to members formed of the Same material is uniformly increased, the radial expansion of the tubular member thereof will correspond exactly to the radial expansion of its end members so that the entire drum, while increasing slightly in diameter will nevertheless retain its original cylindrical form. If, however, the temperature of the end members is not increased as much as that of the tubular portion, the tubular portion will attempt to attain a somewhat greater diameter than the end members but will be unable to uniformly expand because the ends of the tubular member are rigidly joined to the peripheries of the now somewhat smaller end members. By deforming the tubular member, the center portion thereof will expand even though the ends are restrained, thereby causing the casting surface to bow outwardly at the center of the drum. Similarly, if a significant temperature gradient occurs in the tubular member itself along the length of the drum, the hotter portions of the tubular member will tend to expand more than the adjacent cooler portions, thereby Additional stresses tending to distort the casting surface, although probably to a lesser degree, may also result from the tendency of the end members of the drum to assume a dished condition due to temperature variations between the inner and outer 3,l42,86 Patented Aug. 4, 1.964.

'surfaces of these members or between different annular areas thereof.

Since the uniformity of the finished product is determined, at least in part, by the trueness of the cylindrical casting surface of the drum and by the exact location of this casting surface relative to the extrusion hopper and/ or a doctor blade or calendar rolls which determine the film thickness, it is obvious that any distortion in the casting `surface can be fatal to the successful operation of the casting drum to form a lm substantially free of surface imperfections.

Aside from distortion of the drum, still another problem presented by a temperature gradient occurring across the casting face thereof is presented when such a drum is used to produce film or sheeting of certain materials such as polyester plastics. In order to form a tough flexible plastice sheet, the molten polyester material must be chilled quickly to solidify in its tough iiexible and transparent amorphous state rather than in its brittle, cloudy crystalline state. To achieve such rapid cooling, it is essential that the cast film material be in intimate contact with the drum surface. In order to assure such intimate contact and to prevent the possibility of air being drawn under the layer of molten material on the drum, pressure may be applied to the top surface of the molten material at the point where it contacts the drum or a vacuum may be applied between the molten material and the drum at the point of contact. If, however, the surface of the casting drum is too cold, a surface skin of the amorphous plastic is instantly formed which tends to shrink and lift the overlying still molten plastic material out of intimate contact with the cool casting surface. As mentioned previously, the resulting loss of intimate contact reduces the cooling rate and the undesirable crystalline product is obtained.

It is, therefore, very important to maintain a uniform rate of cooling throughout the sheet by minimizing the temperature gradient across the face of the drum.

While the difliculties discussed above have plagued skilled workers in this art for years, I have found that these problems are successfully overcome by the seemingly simple, yet unobvious expedient of insulating the inside surfaces of the end walls of the casting drum so as to retard heat transfer between these walls and the cooling liquid within the drum. Accordingly, a principle object of the invention is to reduce temperature variations throughout a liquid cooled casting drum or chill roll. Another object of the invention is to simplify the construction of a liquid cooled casting drum having means to control the iiow of heat throughout the drum. Still another object of the invention is to reduce distortion in liquid cooled casting drums. A still further object of the invention is to reduce the temperature gradient across the `casting face of such a drum. These and other objects will be apparent from the following description and accompanying drawings, in which:

FIG. '1 is la partial cross sectional view of a casting drum according to the preferred embodiment .of the invention, taken along the vertical center line of the drum;

FIG. 2 is a cross sectional View of one corner of a casting drum showing an alternative insulating arrangement; and

FIG. 3 is a schematic View of a casting drum and asso'- ciated apparatus showing a strip of film being formed thereby.

Referring now to the drawings, FIG. 3 illustrates schematically a casting drum 5 and a hopper 6 which extrudes or tiows a thin strip of molten material onto the cylindrical surface thereof. The drum is rotated in the direction indicated by the arrow, by a mechanism not shown in the drawing, at a speed proportional to the rate at which the molten material is fed from the hopper. As

the molten material is cooled by the drum it solidies into a iiexible film which is separated from the drum by being drawn over roller 7 by a winding mechanism or other processing device.

As shown in FIG. l, the casting drum comprises a tubular member 11, end members 12 and 13, and hub members 14 and 15 adapted to support the drum in appropriate bearings or trunnions, not shown. The end members are welded or otherwise rigidly joined to the corresponding hub members and are rigidly attached to the opposite ends of member 11 by screws 2t) or analogous means to form a closed cylinder. As is evident from the drawing, these members are heavily constructed and cooperate to provide a heavy rigid drum. The cylindrical casting surface 16 of the drum is accurately ground after members 11 through 16 have been assembled, and is provided with peripheral grooves 17 and 18 which form beaded projections 19 and 21 on the lower surface of the cast material 22. These beaded projections and the similar projections 23 and 24 formed on the opposite surface of the lm, by means not shown in the drawing, are employed to transversely stretch the lm after it has left the casting wheel. The size of the beaded projections and the thickness of the cast film are greatly exaggerated in the drawings for purposes of clarity.

In order to circulate cooling water through the interior of the drum, three stationary concentric pipes Z5, 26, and 27 are located at the axis of the drum. The largest of these pipes, 25, is provided with a closed end 28 and is supported in bushings 29 to maintain the pipe in concentric relation to the drum while allowing the drum to rotate thereabout. The opposite end of this pipe is open and extends into hub member 15. Pipe 26, having a closed end 31, is concentrically located within pipe Z and extends through and beyond hub to connect with a water supply line, not shown. A vertical pipe 32 communicating with pipe 26 at approximately the center of the drum passes through the wall of pipe 25 and extends toward the bottom of the drum. Within pipe 26 is concentrically located pipe 27 which communicates through the closed end 31 of the pipe 26 with a vertical pipe 33 extending upwardly through the wall of pipe 25 and terminating in close proximity to member 11 at the top of the drum. Pipe 27 extends beyond the end of pipe 26 and is likewise supported by external means not illustrated. Since pipes 32 and 33 pass through the walls of pipe 2S it is apparent that the three concentric pipes and vertical pipes remain at all times stationary as the drum is rotated about its axis. Cooling water is introduced into the drum through pipe 26 and vertical pipe 32, the lower end of which communicates with a horizontal pipe 36 having closed ends 37 and provided with a slot 38 parallel to and in close proximity to member 11 at the bottom of the drum. This slot distributes the inllowing water across the width of the drum and causes the entire mass of Water to rotate, thereby circulating the water throughout the drum. As cool Water is introduced into the lower peripheral portion of the drum, the water along the drum axis flows into pipe 25 through holes 34 and is conducted through the hollow shaft portion 35 of hub member 15 to an external drainage line through a rotary gland, not shown, surrounding shaft 35 and pipes 26 and 27. In order to maintain the drum full of water and to regulate the rate of flow thereof, adjustable valves, not shown, are provided in the external water supply and drainage lines. Ifipe 33 which communicates with the atmosphere through pipe 27 and a valve, not shown, is provided to allow entrapped air to be removed from the drum.

As mentioned previously, if the rate of heat transfer between the water and all of the internal surfaces of the casting drum is substantially uniform, the ends of the drum will run considerably cooler than member 11 because of the additional heat constantly being absorbed by member 11 from the molten material, only a small part of which heat is conducted directly to the ends of the drum. Likewise the liquid cooled internal surfaces of the drum ends will run considerably colder than the external end surfaces which are not exposed to cooling liquid. The unequal expansion of these members caused by such temperature variations creates stress and distortion in the drum as previously explained. To minimize such temperature variations in the drum, and in accordance With my invention, insulating material 39 is applied to the inner surfaces of the drum ends to greatly reduce the rate at which heat is absorbed from these surfaces by the water. Although various types of insulating material might be employed, foamed polyethylene plastic has proven to be particularly effective for this purpose. In the illustrated embodiment, this insulating material is sandwiched between the ends of the drum and thin metal plates 41 anchored to the drum ends by screws 42. If a form of insulating material is employed that is impervious to water, the insulation may be applied to the drum ends by various other means, as for example by cementing.

With the rate of cooling the ends of the drum reduced by internal insulation, dissipation of the heat conducted directly to the drum ends at their peripheries from member 11 is retarded in proportion to the thickness of the insulation. Therefore, by utilizing insulation of the proper thickness it is possible to greatly reduce temperature variations between member 11 and the drum end members. Since the temperatures of both member 11 and the drum ends are proportional to the amount of heat applied to the casting surface and to the temperature of the coolant, a given thickness of insulation will tend to equalize the temperature of the entire drum over a broader range of operating conditions than can be achieved by the aforementioned approaches. Furthermore, the drum can be adapted to operate under a substantially different temperature by the simple expedient of varying the thickness or type of insulation used, without necessitating dismantling or regrinding the drum.

The heat lost by the outer surfaces of the drum ends to the surrounding air is relatively slight compared to that absorbed from an equivalent internal surface by cooling water. This loss may in some cases nevertheless be signilicant enough to justify the application of insulation to these external surfaces. As shown in the drawings, this insulation, 43, may be applied by means of cement. If it is desirable to increase the temperature of the drum ends beyond that which may be achieved by insulating means alone, or if it is desired to selectively regulate the temperature of the drum ends, additional heat may be applied to the external end surfaces of the drum by infrared heating lamps or other means either in lieu of the external insulation or in areas left uninsulated for this purpose, for example, in an uninsulated annular area as shown at 44.

While the means for controlling the iiow of heat in the drum as illustrated in FIG. 1 and herein described substantially eliminate distortional temperature variations within the drum, the edge portions of member 11 may still run somewhat cooler than the center portion, even though somewhat more heat is applied to the casting surface toward the drum edges by the beaded projections on the film, because some heat is still conducted to the end members and because the internal surface of member 11 is exposed to the cooling water slightly beyond the edges of film 16 on the opposite external surface. While this possible slight temperature gradient has in some cases not been detrimental to the operation of such a drum, a still more uniform temperature across the face of the drum may be achieved by extending the insulation over a greater portion of the inside surface of member 11 to reduce the amount of heat transferred from member 11 to the water in this area as shown in FIG. 2. Since the inner surface of member 11 is slightly recessed in the embodiment illustrated, it is obvious that the internal insulation as shown in FIG. 2 would have to be applied after the drum is assembled. This is desirable in any event to facilitate changing the thickness of the insulation after the drum is assembled, and may be accomplished by forming the internal insulation and, if employed, the sheet metal insulation retaining means, in the form of pie shaped segments, across to the interior of the drum being provided by appropriate man holes as illustrated at 45.

While the illustrated insulation is of substantially uniform thickness, it may prove desirable in some instances to vary the thickness of the insulation at various points within the drum. Thus, an area of the drum that becomes too hot during operation may be cooled by reducing the thickness of the insulation adjacent thereto and, likewise, the temperature of a cool area of the drum may be raised by increasing the thickness of the adjacent insulation.

While the foregoing description refers to the application of the invention to casting drums used in producing polyester film by the melt casting process, it should be understood that the invention is equally applicable to drums used in producing iilm or sheet material of other materials which are thermoplastic at the time the material is cast onto the drum, even though the thermoplastic properties of such materials may subsequently be altered.

Although certain specific embodiments of the invention have been shown and described, various modications thereof are possible. The invention, therefore, is not to be limited to the precise details of construction shown and described but is intended to cover all modifications coming within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. A film casting drum for use in forming a self-supporting film by deposition on the drum periphery of a stream of molten thermoplastic material for cooling to its setting point and comprising: a closed hollow body member having a tubular portion of heat conductive material the external surface of which constitutes the casting surface onto which the stream of molten thermoplastic material is to be deposited and having end walls of heat conductive material closing the ends of said tubular portion and directly connected thereto so as to be in heat conductive relation therewith; means for supporting said body member for rotation about the axis of said tubular portion; and means for iilling said hollow body member with a cooling liquid and thereafter feeding said cooling liquid into and out of said body member while it is rotating to control the temperature of said casting surface; means for reducing to a minimum the temperature gradient between the tubular portion of said body member and said end walls for eliminating possible distortion of said tubular portion, including means insulating said end walls from direct contact with said cooling liquid within said body member to prevent the direct transfer of heat between the two.

2. A iilm casting drum according to claim 1 in which said last-mentioned means comprises a layer of insulating material sandwiched between the inner surface of each of said end walls and a sheet of water impervious material fastened to said body member and protecting said insulating material from contact with said cooling liquid.

3. A film casting drum according to claim 2 in which said tubular portion is wider than the stream of molten thermoplastic material to be deposited onto the external surface thereof whereby the extreme ends of said tubular portion are not contacted by said thermoplastic material, and in which said layer of insulating material adjacent the internal surface of each end wall extends axially of said tubular portion and insulates said extreme ends of said tubular portion from direct contact with said cooling liquid within said body member.

4. A lrn casting drum according to claim 1, in which said means for reducing the temperature gradient between said tubular portion and said end walls includes means for applying additional heat to the external surfaces of the end walls of said body member.

5. A construction according to claim l including means for distributing liquid ilowing into said hollow body member and across the internal cylindrical surface of said tubular portion in the lower portion thereof and for directing the inflowing liquid tangentially of said tubular portion, and means for removing said liquid from said hollow body member along the axis of said tubular portion.

References Cited in the iile of this patent UNITED STATES PATENTS 1,001,072 Du Pont Aug. 22, 1911 1,008,607 Mellinger et al. Nov. 14, 1911 1,235,716 Mooney Aug. 7, 1917 1,318,464 Schweizer Oct. 14, 1919 1,640,855 Sclick Aug. 30, 1927 2,068,181 Hurxthal Jan. 19, 1937 2,169,625 Weiss et al. Aug. 15, 1939 2,292,760 Kath Aug. 11, 1942 2,367,578 Helin Jan. 16, 1945 2,412,733 Hornbostel Dec. 17, 1946 2,413,567 Hornbostel Dec. 31, 1946 2,555,309 Beam June 5, 1951 FOREIGN PATENTS 447,658 France Nov. 4, 1912 496,543 Germany Apr. 25, 1930 

1. A FILM CASTING DRUM FOR USE IN FORMING A SELF-SUPPORTING FILM BY DEPOSITION ON THE DRUM PERIPHERY OF A STREAM OF MOLTEN THERMOPLASTIC MATERIAL FOR COOLING TO ITS SETTING POINT AND COMPRISING: A CLOSED HOLLOW BODY MEMBER HAVING A TUBULAR PORTION OF HEAT CONDUCTIVE MATERIAL THE EXTERNAL SURFACE OF WHICH CONSTITUTES THE CASTING SURFACE ONTO WHICH THE STREAM OF MOLTEN THERMOPLASTIC MATERIAL IS TO BE DEPOSITED AND HAVING END WALLS OF HEAT CONDUCTIVE MATERIAL CLOSING THE ENDS OF SAID TUBULAR PORTION AND DIRECTLY CONNECTED THERETO SO AS TO BE IN HEAT CONDUCTIVE RELATION THEREWITH; MEANS FOR SUPPORTING SAID BODY MEMBER FOR ROTATION ABOUT THE AXIS OF SAID TUBULAR PORTION; AND MEANS FOR FILLING SAID HOLLOW BODY MEMBER WITH A COOLING LIQUID AND THEREAFTER FEEDING SAID COOLING LIQUID INTO AND OUT OF SAID BODY MEMBER WHILE 